Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NOx storage catalyst which stores NOx which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges a fuel addition valve in the engine exhaust passage upstream of the NOx storage catalyst, which arranges an air-fuel ratio sensor inside the engine exhaust passage downstream of the NOx storage catalyst, and which makes the air-fuel ratio of the exhaust gas which flows into the NOx storage catalyst rich by feeding fuel in the engine exhaust passage from the fuel addition valve when NOx should be released from the NOx storage catalyst. (see, for example, Patent Literature 1).
In this regard, in this internal combustion engine, if the fuel addition valve feeds more than the necessary excess hydrocarbons, the amount of hydrocarbons which slip through the NOx storage catalyst increases. In this case, if it were possible to detect the amount of slip through of the hydrocarbons, it would be possible to reduce the amount of hydrocarbons which are wastefully consumed. However, at the present, it is impossible to detect such an amount of slip through of hydrocarbons by a simple method.